You can't link a guest order to your main account.
It's always a good idea to log in and use your main account - when you buy as a guest, it can be difficult to communicate with the seller should you need to, and you can't leave feedback.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
